Gaining Insights into the Interplay between Optical and Magnetic Properties in Photoexcited Coordination Compounds

We describe early and recent advances in the fascinating field of combined magnetic and optical properties of inorganic coordination compounds and in particular of 3d-4f single molecule magnets. We cover various applied techniques which allow for the correlation of results obtained in the frequency and time domain in order to highlight the specific properties of these compounds and the future challenges towards multidimensional spectroscopic tools. An important point is to understand the details of the interplay of magnetic and optical properties through performing time-resolved studies in the presence of external fields especially magnetic ones. This will enable further exploration of this fundamental interactions i. e. the two components of electromagnetic radiation influencing optical properties.

Heterometallic clusters based on an uncommon asymmetric "V-shaped" [Fe3+(μ-OR)Ln3+(μ-OR)2Fe3+]6+ (Ln = Gd, Tb, Dy, Ho) structural core and the investigation of the slow relaxation of the magnetization behaviour of the [Fe2Dy] analogue

The synthesis, crystal structures, Mössbauer spectra and variable temperature dc and ac magnetic susceptibility studies of a new family of trinuclear heterometallic Fe³⁺/Ln³⁺ complexes, [Fe₂Ln(PhCO₂)₃((py)₂CO₂)((py)₂C(OMe)O)₂(NO₃)Cl] (Ln = Gd (1/Gd), Tb (1/Tb), Dy (1/Dy), and Ho (1/Ho)), where (py)₂CO₂^2- and (py)₂C(OMe)O^- are the anions of the gem-diol and hemiketal derivatives of di-2-pyridyl ketone, are reported. Compounds 1/Ln are based on an asymmetric "V-shaped" [Fe³⁺(μ-OR)Ln(μ-OR)₂Fe³⁺]⁶⁺ structural core formed from the connection of the two terminal Fe³⁺ centers to the central Ln³⁺ ion either through one or two alkoxide groups originating from the alkoxide-type bridging ligands. Direct current magnetic susceptibility studies reveal the presence of weak antiferromagnetic interactions between the Fe³⁺ ions. Alternating current magnetic studies indicate the presence of a slow-magnetic relaxation process in 1/Dy with an energy barrier U_eff = 6.7 (±0.3) K and a pre-exponential factor, τ₀ = 2.2 (±0.4) × 10^-7 s. The electronic, magnetic and relaxation properties of the complexes were further monitored by variable temperature ⁵⁷Fe Mössbauer spectroscopy. At T > 80 K the spectra from the complexes comprise two quadrupole doublets the hyperfine parameters of which reflect the distinct coordination environment of the two Fe³⁺ terminal sites. At T < 20 K, the Mössbauer spectra for 1/Dy are affected by magnetic relaxation effects. At 1.5 K, the spectrum of 1/Dy comprises well defined magnetic sextets indicating relaxation times slower than the characteristic time of the Mössbauer technique (10^-7 s) in agreement with the dynamic magnetic measurements. 1/Gd exhibits broad unresolved magnetic sextets at 1.5 K indicating that the spin relaxation time is of the order of the Mössbauer characteristic time at this temperature. For 1/Tb, 1/Ho the Mössbauer spectra exhibit slight broadening even at the lowest available temperature consistent with magnetic relaxation times less than 10^-7 s.

Recent advances in lanthanide coordination polymers and clusters with magnetocaloric effect or single-molecule magnet behavior

Molecular magnetorefrigerant materials for low-temperature magnetic refrigeration and single-molecule magnets for high-density information storage and quantum computing have received extensive attention from chemists and magnetic experts. Lanthanide ions with unique magnetic properties have always been considered as ideal candidates for the construction of such materials. This frontier article focuses on Gd^III-based molecular magnetorefrigerants and lanthanide-based single-molecule magnets and highlights the most significant advances.

From the {FeIII 2 Ln2 } Butterfly's Perspective: the Magnetic Benefits and Challenges of Cooperativity within 3 d-4 f Based Coordination Clusters

In this Review we discuss the tuning handles which can be used to steer the magnetic properties of Fe^III‐4 f “butterfly” compounds. The majority of presented compounds were produced in the context of project A3 “Di‐ to tetranuclear compounds incorporating highly anisotropic paramagnetic metal ions” within the SFB/TRR88 “3MET”. These contain {Fe^III₂Ln₂} cores encapsulated in ligand shells which are easy to tune in a “test‐bed” system. We identify the following advantages and variables in such systems: (i) the complexes are structurally simple usually with one crystallographically independent Fe^III and Ln^III, respectively. This simplifies theory and anaylsis; (ii) choosing Fe allows ⁵⁷Fe Mössbauer spectroscopy to be used as an additional technique which can give information about oxidation levels and spin states, local moments at the iron nuclei and spin‐relaxation and, more importantly, about the anisotropy not only of the studied isotope, but also of elements interacting with this isotope; (iii) isostructural analogues with all the available (i. e. not Pm) 4 f ions can be synthesised, enabling a systematic survey of the influence of the 4 f ion on the electronic structure; (iv) this cluster type is obtained by reacting [Fe^III₃O(O₂CR)₆(L)₃](X) (X=anion, L=solvent such as H₂O, py) with an ethanolamine‐based ligand L′ and lanthanide salts. This allows to study analogues of [Fe^III₂Ln₂(μ₃‐OH)₂(L′)₂(O₂CR)₆] using the appropriate iron trinuclear starting materials. (v) the organic main ligand can be readily functionalised, facilitating a systematic investigation of the effect of organic substituents on the ligands on the magnetic properties of the complexes. We describe and discuss 34 {M^III₂Ln₂} (M=Fe or in one case Al) butterfly compounds which have been reported up to 2020. The analysis of these gives perspectives for designing new SMM systems with specific electronic and magnetic signatures

A new family of Fe4Ln4 (Ln = DyIII, GdIII, YIII) wheel type complexes with ferromagnetic interaction, magnetocaloric effect and zero-field SMM behavior

Observation of ferromagnetic interactions and single molecule toroic (SMT) behavior in Fe4Ln4 wheel complexes.

A family of planar hexanuclear CoLn clusters with lucanidae-like arrangement and single-molecule magnet behavior

A family of planar hexanuclear CoLn clusters with the hydroxyl and nitrogen rich ligand 2-[bis(pyridin-2-ylmethyl) amino]-2-(hydroxymethyl)propane-1,3-diol (H₃L), formulated as [Co₄Ln₂(μ₃-O)₂(μ-N₃)₂(OH)₂(H₂O)₂(HL)₄]·(CH₃CO₂)₂·20H₂O [Ln = Dy (1), Gd (2), Tb (3), Eu (4) and Ho (5)], have been synthesized and structurally characterized. They are isomorphous and feature a lucanidae-like arrangement. They are the first examples of 3d-4f clusters with the H₃L ligand. In addition, the magnetic properties of 1-5 have been investigated and the single-molecule magnet (SMM) behaviour is observed for 1.

Mechanism of magnetisation relaxation in {MIII2DyIII2} (M = Cr, Mn, Fe, Al) "Butterfly" complexes: how important are the transition metal ions here?

We describe the synthesis, characterisation and magnetic studies of four tetranuclear, isostructural "butterfly" heterometallic complexes: [MIII2LnIII2(μ3-OH)2(p-Me-PhCO2)6(L)2] (H2L = 2,2'-((pyridin-2-ylmethyl)azanediyl)bis(ethan-1-ol), M = Cr, Ln = Dy (1), Y (2), M = Mn, Ln = Dy (3), Y (4)), which extend our previous study on the analogous 5 {Fe2Dy2}, 6 {Fe2Y2} and 7 {Al2Dy2} compounds. We also present data on the yttrium diluted 7 {Al2Dy2}: 8 {Al2Dy0.18Y1.82}. Compounds dc and ac magnetic susceptibility data reveal single-molecule magnet (SMM) behaviour for complex 3 {Mn2Dy2}, in the absence of an external magnetic field, with an anisotropy barrier U eff of 19.2 K, while complex 1 {Cr2Dy2}, shows no ac signals even under applied dc field, indicating absence of SMM behaviour. The diluted sample 8 {Al2Dy0.18Y1.82} shows field induced SMM behaviour with an anisotropy barrier U eff of 69.3 K. Furthermore, the theoretical magnetic properties of [MIII2LnIII2(μ3-OH)2(p-Me-PhCO2)6(L)2] (M = Cr, 1 or Mn, 3) and their isostructural complexes: [MIII2DyIII2(μ3-OH)2(p-Me-PhCO2)6(L)2] (M = Fe, 5 or Al, 7) are discussed and compared. To understand the experimental observations for this family, DFT and ab initio CASSCF + RASSI-SO calculations were performed. The experimental and theoretical calculations suggest that altering the 3dIII ions can affect the single-ion properties and the nature and the magnitude of the 3dIII-3dIII, 3dIII-DyIII and DyIII-DyIII magnetic coupling, thus quenching the quantum tunneling of magnetisation (QTM) significantly, thereby improving the SMM properties within this motif. This is the first systematic study looking at variation and therefore role of trivalent transition metal ions, as well as the diamgnetic AlIII ion, on slow relaxation of magnetisation within a series of isostructural 3d-4f butterfly compounds.

A W-Shaped Ga-Dy-Dy-Dy-Ga Cluster: Synthesis, Characterization, and Magnetic Properties

A W-shaped {Ga-Dy-Dy-Dy-Ga} coordination cluster, [Ga₂ Dy₃ (L)₆ (MeO)₂ (MeOH)₃ ]⋅NO₃ ⋅7 MeOH (1) with H₂ L, (E)-2-(2-hydroxy-3-methoxy-benzylideneamino)phenol was synthesized, characterized, and the magnetic properties investigated. The magnetic measurements indicate that the compound shows single-molecule magnet (SMM) behavior under zero dc field. This suggests that the combination of Ga^III ions and Dy^III ions (4 f) is a successful strategy to assemble SMM clusters. The combination of these metal ions also offers prospects in dual functionality therapeutics for medical applications.

Butterfly M2 IIIEr2 (MIII = Fe and Al) SMMs: Synthesis, Characterization, and Magnetic Properties

The reaction of N-(2-pyridylmethyl)iminodiethanol (H₂L, pmide), FeCl₂·4H₂O or AlCl₃·6H₂O with ErCl₃·6H₂O and p-Me-PhCO₂H in the ratio of 2:1:1:4 in the presence of Et₃N in MeOH and MeCN yielded compounds [Fe₂Er₂(μ₃-OH)₂(pmide)₂(p-Me-PhCO₂)₆]·2MeCN (1) and [Al₂Er₂(μ₃-OH)₂(pmide)₂(p-Me-PhCO₂)₆]·2MeCN (2). These two complexes are isostructural, possessing a planar butterfly motif with the Er^III ions in the wingtip positions. Both compounds show single molecule magnet (SMM) behavior. For the [Al₂Er₂] compound, the slow relaxation of the magnetization under zero applied direct current (dc) field does not show maxima, but the relaxation processes could be analyzed using an applied dc field of 1000 Oe. In-depth alternating current measurements under different dc fields on the [Fe₂Er₂] compound reveals that the Fe-Fe and Fe-Er interactions speed up the relaxation and decrease the energy barrier height of the SMM in comparison with the [Al₂Er₂] case.

A Three-Pronged Attack To Investigate the Electronic Structure of a Family of Ferromagnetic Fe4Ln2 Cyclic Coordination Clusters: A Combined Magnetic Susceptibility, High-Field/High-Frequency Electron Paramagnetic Resonance, and 57Fe Mössbauer Study

We present the synthesis, structure, magnetic properties, as well as the Mössbauer and electron paramagnetic resonance studies of a ring-shaped [Fe^III₄Ln^III₂(Htea)₄(μ-N₃)₄(N₃)₃(piv)₃] (Ln = Y 1, Gd 2, Tb 3, Dy 4, Ho 5, Er, 6) coordination cluster. The Dy, Tb, and Ho analogues show blocking of the magnetization at low temperatures without applied fields. The anisotropy of the 3d ion and the exchange interaction between 3d and 4f ions in Fe₄Ln₂ complexes are unambiguously determined by high-field/high-frequency electron paramagnetic resonance measurements at low temperature. Ferromagnetic exchange interaction J_Fe-Ln is found which decreases upon variation of the Ln ions to larger atomic numbers. This dependence is similar to the behavior shown in the effective barrier values of complexes 3-5. Further information about the anisotropy of the Ln³⁺ ions was gathered with ⁵⁷Fe Mössbauer spectroscopy, and the combination of these methods provides detailed information regarding the electronic structure of these complexes.

The role of coordinated solvent on Co(ii) ions in tuning the single molecule magnet properties in a {CoII2DyIII2} system

The anisotropy of a single Dy^III can be steered through subtle changes in the coordination geometry of the Co^II ions in CoII2DyIII2 butterfly compounds resulting from changing the coordinated solvent molecule on the Co^II ion.